1. Technical Field
Described herein is a nonvolatile semiconductor memory apparatus.
2. Related Art
In recent years, electrically writable and erasable nonvolatile semiconductor memory apparatuses have been developed to provide higher performances. Such nonvolatile semiconductor memory apparatuses include, for example, EEPROMs (Electrically Erasable Programmable Read Only Memories), and there are two well-known types of EEPROMs: a floating gate type (hereinafter also referred to as the FG type) and a MONOS (Metal Oxide Nitride Oxide Semiconductor) type.
The fundamental structure of the FG type is characterized by a stacked structure formed with a control gate electrode, an interelectrode insulating film (also called an interpoly insulating film), a floating gate electrode, a tunnel insulating film (a SiO2 film), and a substrate stacked in this order from the top. When a positive high voltage is applied to the control gate electrode, electrons can be injected (written) into the floating gate electrode from the substrate side. When a negative high voltage is applied, electrons can be removed (erased) from the floating gate electrode toward the substrate. Ideally, the electrons once written into the floating gate electrode remain in the floating gate electrode, unless an erasing operation is performed. Accordingly, the apparatus functions as a nonvolatile memory apparatus.
The fundamental structure of the MONOS type has a stacked structure formed with a control gate electrode, a block insulating film, a trapping film, a tunnel insulating film, and a substrate stacked in this order from the top. When writing is performed in this structure, electrons are injected by applying a high voltage, as in the FG-type structure. However, the electrons are stored in the trapping film. Erasing is performed not only by removing electrons but also by injecting holes, which differs from the erasing operation in the FG-type structure. In this manner, the stored electrons and the holes cancel each other, so as to realize erasing.
To improve the performance of a nonvolatile semiconductor apparatus, the erase efficiency may be increased. To increase the erase efficiency, a high voltage is applied to the tunnel insulating film so as to inject a large number of holes in a short period of time. In other words, the leakage current is increased. It is known that the relationship between the leakage current J flowing in the tunnel insulating film and the electric field Eox (=Vox/Tox) stays almost the same even if Tox varies, as long as Vox is sufficiently large. Here, Tox represents the film thickness of the tunnel insulating film, and Vox represents the voltage applied to the film. The reason is that the mechanism of the leakage current is dominated by the so-called Fowler-Nordheim (F-N) current Jfn, which is approximately expressed by the following equation (1):
                              J          fn                =                              AE            ox            2                    ⁢                      exp            ⁡                          (                              -                                  B                                      E                    ox                                                              )                                                          (        1        )            
In the case of a hole current, A and B represent the constants that depend on the tunneling mass of the holes in the tunnel insulating film and the barrier height felt by the holes. As can be seen from this equation, the leakage current is not affected by the film thickness Tox, but is determined by the electric field Eox. The possible common specs of the hole leakage current required in the tunnel insulating film indicate that the leakage current is 1.0×10−16 A/cm2 or less in a low electric field of 3 MV/cm, and the leakage current is 1.0×10−5 A/cm2 or greater in a high electric field of 13 MV/cm. Accordingly, to increase the erase efficiency, the electric field Eox may be made higher. However, a higher electric field results in a higher voltage that is undesirable. A higher voltage adversely affects the reliability of the tunnel insulating film.
When a high voltage is applied to the tunnel insulating film, a phenomenon called stress-induced leakage current (hereinafter also referred to as SILC) is caused, and the leakage current is increased with a low voltage. This phenomenon is considered to be caused by defects formed in the tunnel insulating film. If the voltage applied to the tunnel insulating film is high, the electrons tunneling through the bandgap of the tunnel insulating film reach the conduction band of the tunnel insulating film. When moving on to the anode side, the electrons reaching the conduction band have a large energy. Because of the energy, impact ionization is caused, and holes are generated. The holes generated in this manner travel in the opposite direction from electrons, and form defects in the tunnel insulating film. The defects trigger SILC. Therefore, it is preferable that the leakage current is increased so as to achieve higher erase efficiency, without an increase in the electric field Eox. For example, it is ideal that a leakage current of 1.0×10−5 A/cm2 or greater is obtained when the electric field Eox is 13 MV/cm, or a leakage current of 1.0×10−5 A/cm2 is obtained when the electric field Eox is lower than 13 MV/cm.
To realize such a structure, the inventors of the present invention suggested a method for forming a trap (a site that captures and releases electrons) at a shallow level from the conduction band of a tunnel insulating film (a SiO2 film) (see JP-A 2008-147390(KOKAI), for example). The tunnel insulating film disclosed in JP-A 2008-147390(KOKAI) has a three-layer stacked structure in which an insulating layer not having a trap level is interposed between two insulating layers each having a trap level. In a low electric field, the tunneling current in the tunnel insulating film is restricted to the same amount as that in an insulating film not having a trap level.
In a high electric field, on the other hand, electrons tunnel through the trap level. Accordingly, the tunneling probability becomes higher than that in an insulating film not having a trap level, and the leakage current also becomes greater. In this manner, the write efficiency can be made higher without an increase in the electric field Eox. This is an ideal feature for a tunnel insulating film. The above is a case of an electron leakage current Je. In a case of a hole leakage current Jh, Ge is added, for example, to a shallow level from the valence band of the tunnel insulating film (a SiO2 film), so as to form a trap (a site that captures and releases holes). In this manner, the erase efficiency can be made higher, like the write efficiency.
If the voltage can be made even lower, SILC can be restricted, and the reliability of the tunnel insulating film can be made even higher. To do so, the tunnel insulating film should be made thinner. However, if the film thickness of the tunnel SiO2 film is reduced to 5 nm or less, the D-T (Direct Tunneling) current caused by electrons becomes dominant in a low electric field of 3 MV/cm in the electric field Eox, and the above mentioned specs required in the tunnel insulating film are not fulfilled. Therefore, the film thickness of the tunnel insulating film cannot be reduced to 5 nm or less.
Attention should be paid to whether or not the leakage current Je fulfills the specs. Since the barrier height against holes tends to be higher than the barrier height against electrons, the leakage current Jh generated by holes is lower than the leakage current Je generated by electrons in any electric field (Jh<Je), or the leakage current Je is always dominant, in a case where the structure of the insulating film is symmetrical in the film thickness direction. Accordingly, in a low electric field, the leakage current Jh is automatically restricted to a small amount, as long as the leakage current Je is restricted to a small amount.
In terms of the electric properties (the J-Eox characteristics), a tunnel insulating film having a trap level is basically the same as an insulating film not having a trap level in a low electric field, and therefore, the film thickness of the tunnel insulating film having a trap level cannot be reduced to 5 nm or less.
Meanwhile, as a technique for reducing leakage current, the use of a high-dielectric film is known (see Japanese Patent No. 3,357,861, for example). According to Japanese Patent No. 3,357,861, the leakage current in a low electric field cannot be reduced merely by the use of a single-layer high-dielectric film, and the leakage current in a high electric field cannot be increased. However, if a stacked structure includes a high-dielectric film and a low-dielectric film at an appropriate ratio, the leakage current can be reduced in a low electric field, and can be increased in a high electric field. The equivalent oxide thickness (EOT) of the tunnel insulating film having such a stacked structure can be reduced, and the voltage can be lowered accordingly. Generally there is a tendency that the higher a dielectric constant, the lower the barrier height. The tendency leads to the problem of an increase in current due to the thermal excitation of electrons; however, this problem can be solved by the stacked structure in Japanese Patent No. 3,357,861. Thus, the leakage current can be reduced by the use of a stacked structure including a low-dielectric film that tends to have a large barrier height.
In this stacked structure, the EOT can be reduced to 5 nm or less. Accordingly, a high leakage current of 1.0×10−5 A/cm2 generated by holes and normally required for erasing can be obtained with a low voltage Vins. However, with the electric field Eox converted into an oxide film (=Vins/EOT) being taken into account, it is necessary to have an electric field higher than 13 MV/cm. A stacked structure formed with a low-dielectric film and a high-dielectric film has the advantage that the film thickness can be reduced while the leakage current in a low electric field is restricted to a small amount, but also has the drawback that a high electric field Eox is required to obtain a high current necessary for erasing.
As described above, by the conventional art, the EOT of a tunnel insulating film cannot be reduced to 5 nm or less while the specs of the leakage current required in a nonvolatile semiconductor memory apparatus are fulfilled. Since the film thickness cannot be made smaller, it is necessary to apply a high voltage when data writing or erasing is performed. As a result, defects are formed in the insulating film, leading to SILC.